Determining the boundaries, structure and volume of buried shell matrix deposits using ground-penetrating radar: A case study from northern Australia

Abstract

Ground-penetrating radar (GPR) is used in this study to delineate the extent and internal structure of a large late Holocene buried shell matrix site at Thundiy, Bentinck Island, northern Australia. Shell matrix sites comprise a key component of the coastal archaeological record. The extensive nature of many shell matrix sites presents challenges for archaeological sampling regimes. While large-scale excavation is undesirable and impractical, limited test pits often represent only a tiny fraction of large shell deposits and are rarely considered representative. This study transforms GPR data into three-dimensional models which form the basis of deposit volume estimates. Volume estimates are evaluated against excavation data to test their accuracy. Results demonstrate that this novel methodology can generate accurate three-dimensional representations of buried shell matrices and highly accurate volume estimations with error margins of 3.5% ± 7%. It is recommended, though, that more inclusive error margins of 19.5% ± 17% are used to account for potential error, especially where results cannot be verified. This greater understanding of the extent and structural variability of deposits can be utilised to create robust sampling strategies for excavation. The methodology could also be further employed to enhance comparative regional studies and to add to conservation and management practices of buried shell matrix sites. If applied more widely this methodology will not only benefit our understanding of shell matrix deposits but also the wider archaeological record of coastal regions worldwide.

Estimating Volumes of Coastal Shell Midden Sites Using Geometric Solids

“…Through the application of GIS and Python scripting, volume calculations can be scaled across many sites simultaneously, with automated calculations allowing for examination of data on a landscape level. As discussed, empirical observations remain vital to refining this 3D shape characterization through systematic coring (Letham et al 2017) and, perhaps, application of geophysical methods (Cariou et al 2018; Kenady et al 2018), although heavily forested sites on the Northwest Coast can be challenging (but see Urban and Carter [2021] for a recent successful application). Coring and auguring provide a good balance of time and effort (Cannon 2000; Duffield et al 2020; Martindale et al 2009; McKechnie 2014), and greater attention to this methodology will allow for refinement of accurate volume calculations in different cultural regions and coastal settings.…”

Creating volume estimates for buried shell deposits: A comparative experimental case study using ground‐penetrating radar (GPR) and electrical resistivity under varying soil conditions

“…This study employs both GPR and electrical resistivity to transform the survey results into volume estimates and 3D models of the deposits using a methodology that is detailed in Kenady, Lowe, and Ulm () and Kenady (). The methods were tested under experimental conditions so that results from the modelling could be compared to the actual in‐ground deposits to test each geophysical method's accuracy in generating volume calculations and 3D models of the buried shell deposits.…”